1. Field of the Invention
This invention concerns punching tools and actuators therefor, particularly adapted for piercing holes in die formed sheet metal workpieces.
2. Description of the Prior Art
Die forming of sheet metal in a press is a very common process for manufacturing such parts as automotive body parts, appliance panels, etc. It is often necessary to form holes in the formed workpiece by a secondary piercing operation. One method for carrying out such secondary operation has been the mechanical driving of a punch with a cam mechanism operated by the die press operation, with return springs stripping the punch after the workpiece has been pierced.
A disadvantage of this approach is that there often arises situations where holes are required only with some of the die-formed workpieces, or holes are needed in different locations on different runs of workpieces.
In these cases, disabling or activating a particular punching mechanism requires time consuming installation or removal of the punch mechanisms.
The press ram stroke necessary for operation of the punch mechanisms makes it impossible to carry out forming and piercing in the same station, necessitating a separate station and further part handling.
Sometimes the hole locations necessitate particularly complicated arrangements to execute piercing the holes with a mechanical drive.
Another approach has been to provide hydraulically operated punches, with these units installed on the bed of a fixture. This also requires an additional station, necessitating more plant floor space and further part handling to load and unload the same.
Sometimes, die designers have used off-the-shelf cylinders, which results in a bulky, complex installations.
While specially designed hydraulic piercing devices have been developed to provide simpler and more compact installations, these have not alleviated the need for a separate station.
Another problem in hydraulic punch installations has involved the elimination of air bubbles which can require disassembly and bleeding through the fittings, as there is no other way for air bubbles to be eliminated due to the long lines between the reservoir and the punch actuators.
A further problem is encountered in accumulator type hydraulic pressure sources, in that there is sometimes insufficient available volume, despite a sensed adequate pressure level.
The hydraulic cylinder installations have been bulky in part because of the need to provide hydraulic connections to each side of the cylinder piston, necessitating a fitting projecting from the side of the cylinder.
Accordingly, it is an object of the present invention to provide an arrangement for punching formed workpieces which is readily programmable and which allows punching to be carried out in the same station as the forming step.
It is a further object to provide a hydraulical punch and actuator assembly for such arrangement which is particularly compact.
It is yet another object to provide a hydraulic punch actuator which can eliminate air bubbles without bleeding of the lines, and an accumulator type pressure source in which it is assured that a sufficient volume of pressurized fluid is available to operate the punch actuators.